Corrosion reduction in heat exchangers



Dec. 17,1946.

w. F. HARLOW 2,412,809

CORROSION REDUCTION IN HEAT EXCHANGERS Fil ed Aug. 8, 1945 A a/vc/WALJf/c Rear/N6 FLU/0 Marl/2.5.

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mmvrm Walter]? Harlan 7 Patented Dec. 17, 1946 CORROSION REDUCTION INHEAT EXCHANGERS Walter F. Harlow, Quarndon, England, assignor toCombustion Engineering Company, Inc., New

' York, N. Y.

Application August 8, 1945; Serial No. 609,605 In Great Britain June 21,1944 1 The invention relates to reducing corrosion and clogging in heatexchange apparatus particularly a in economizers and/or air heatersprovided in steam boiler plants for recovering heat from flue gasesafter they have left the boilerproper.

. In such plants, more especially those operating with a high finalsteam temperature say of 800 F. upwards and fired by mechanical stokers,serious difficulty is frequently encountered with choking of the gaspassages of the economizer and/or air heater which greatly hinders theoper ation of the plant and often results in corrosion and destructionof the heating surfaces.

Much research has been carried out on this problem and it is fairly wellknown that this choking and corrosion is largely due to acidiccondensate forming on the heat absorbing surfaces of economizers or airpreheaters although these surfaces are at a temperature above that atwhich condensation should normally occur; that is to say, it is a resultof the existence of an abnormally high dew point temperature of the flueregistered on certain plants whereas in other cases the more normalfigure of 1.30 F. was obtained. It was also shown that in the plantswhere the high dew point temperatures were recorded there was a greateramount of sulphur trioxide in the flue gases than in plants where thedew point temperature was lower. It was concluded Claims. Cl. 257-1)acid or direct combination with the iron of the heat absorbing surfaceresults in the corrosion and depositsdescribed. I have moreoverdiscovered that the sulphur trioxide is formed as a result of thepassage of the flue gases containing $02 over the surfaces of heatabsorbing elements in advance of the economizer or air preheater such asthe superheater and/or boiler tubes,

Y the amount produced depending on the nature that this factor accountedfor the difference since the sulphur trioxide'would combine with thewater vapor which is also present, forming sulphuric acid vapor; andsulphuric acid having a higher boiling point, depending on itsconcentration, than water would consequently condense at highertemperatures.- The reason for the existence in some cases of a greateramount of sulphur trioxide, however, was not determined.

The object of the invention is to minimize such corrosion and cloggingby removing the causes from which this trouble arises.

The single figure of. the accompanying drawing is a diagrammaticelevational view of an illustrative steam generating boiler in which thepresent invention is practiced.

The present applicant has studied the problem over a period of fifteenyears and has confirmed by a great many experiments on operating plantsthat the difficulties referred to are caused by relatively excessiveamounts of' sulphur trioxide in the fiue gases which by deposition ofsulphuric and temperature of these surfaces ahead of the heat exchanger.I have found by laboratory investigations and confirmed by observationstaken on plants that this is principally due to the catalytic effect ofa film of ferric oxide (F8203) and to a lesser extent, of therefractory-like scale which frequently forms on iron or steel surfacesof heat absorbing elements such as superheaters; also that the'extent ofthis action while comparatively slight at surface temperatures of say upto 600 F. afterwards increases and in the case of ferric oxide becomesvery much greater,

between 800 F. and 1200" F.

One method by which I eliminate this catalytic action is to coat thesesurfaces of iron with the black magnetic oxide (F8304) such as isobtained by heating the iron to a temperature of say 1600 in air or fluegas or to 1200" F.'in an atmosphere of steam, and then the heated irondoes not produce the catalytic effect referred to.

Another method is to coat the surfaces of iron or of refractory or ofscale which are catalytic with certain fine dusts (say of the finenessof 10 micron or less) if only to a thickness of say one thousandth of aninch; the surfaces then lose their catalytic properties, providing ofcourse the dust particles adhere to the surfaces. For in- I stance ifthe surfaces, preferably when heated, are

sprayed with or even merely dipped in an aqueous suspension of very finepulverized fuel ash containing say 10% dust or a suspension of" limehydrate containing as little as 3% lime the catalytic action is almostcompletely eliminated.

Applicant has also found that if the catalytic surfaces referred toabove are sprayed with, or even dipped (preferably when in a heatedcondition) in a concentrated aqueous solution of sodium carbonate(commercial soda ash is quite suitable) the catalytic effect is alsopractically eliminated. In practice I have found that by coatings'uperheater tubes with an adherent film of lime before erection in theboiler a considerable advantage has been gained.

In the case of a boiler in which the fouling of the air heater was suchthat its availability was only 384, hours, this was extended to 1402hours auaeoa the coal even if the normal amount of sulphur trioxide only(i. e., neglecting the sulphur dioxide ordinarily produced) is takeninto account, showing that the process really prevents the acid from 1forming and does not merely neutralize it after formation: in fact noneof the methods of treatment disclosed herein are intended to chemicallyneutralize disadvantageous constituents of flue gases. 7

I cover the superheater tubes or other heating surfaces commonly usedahead of the air preheater or economizer with a film of non-catalyticmaterialby spraying them with or otherwise applying a liquid having anadherent suspension of lime, sodium carbonate, finely pulverized coalash I or other suitable material, preferably when the surfaces are in aheated condition. The spray may be produced by delivering the liquidsuspension through a fine nozzle from a container under pressure or byintroducing the suspension into an air or steam pipe so that the coatingmaterial is dispersed, and carried onto the heating heater, as shownsolely by way of illustration in the drawing. It is proposed to coat thesurfaces from time to time as may be necessary whilst the boiler isinoperation, and/or to apply the coat-'.

ing before use is made of the boiler.

It has been found that the deposits of siliceous material whichsometimes form on the heating surfaces of boilers and superheaters arealso absorbing elements that 4 s are located in advance of said heatexchanger in the direction of gas flow which method comprises: applyingan adherent liquid solution containing sodium carbonate to thesurfaces'of said heat absorbing elements.

3. The method of minimizing corrosion by action of sulphuric acid on themetallic surfaces of a heat exchanger under circumstances whereincombustion gases containing 80: are subjected to catalytic action inpassing over hot iron or steel heat absorbing elements that are locatedin advance of said heat exchanger in the direction of gas flow. whichmethod comprises: periodically spraying the surfaces of said heatabsorbing lements with an aqueous solution containing sodium carbonateequivalent to approximately one pound per'hundred square feet of surfaceto form a non-catalytic coating thereon. 4. The method of minimizingcorrosion by action of sulphuric acids on the metallic surfaces of aheat exchanger under circumstances wherein combustion gases containingare subjected to catalyticaction in passing over iron or steelsuperheater elements that are located in advance of said heat exchangerin the direction of gas flow and heated to a temperature in the range800 to 1200 E, which method comprises: periodically spraying thesurfaces of said superheater elements during operation thereof with anaqueous solution containing sodium carbonate to form a non-catalyticsurface thereon.

5.' The'method of minimizing corrosion by action of sulphuric acid on"the metallic surfaces Y in advance of said heat exchanger in thedireccatalytic but to a lesser extent although they become effectivewhen the encrustation is substantial. The methods described may beeffectively employed for avoiding this undesirable condition.

This application is a continuation in part of that died in my name onOctober 11,1944, under Serial No. 558,291.

What I claim is:

1. The method of minimizing corrosionby action of sulphuric acid on themetallic surfaces of a heat exchanger under circumstances. whereincombustion gases containing, SO: are subjected to catalytic action inpassing over hot iron or steel heat absorbing elements that are locatedin advance of said heat exchanger in the direction of gas flow, whichmethod comprises: treating the gas contacted surfaces of said heatabsorbing elements with a material which is non-catalytic in thepresence'of the sulphur dioxide in the furnace gases to thereby avoidits conversion to sulphur trioxide and thesubsequent formation of a heatexchanger under circumstances wherein combust on gases containing 802are subjected to catalytic action in passing over iron or st el heattion of sulphuric acid on the metallic surfaces tion of gas flow andheated to a temperature in the range 860 to 1200". R, which methodcomprises: periodically spraying the surfaces of said superheaterelements with an aqueous solution containing sodium hydrate to form anon-catalytic surface thereon.

6. The method of minimizing corrosion by action of sulphuric acid on themetallic surfaces of a heat exchanger under circumstances whereincombustion gases containing SO: are subjected to catalytic action inpassing over hot iron or.

steel heat absorbing elements that are located in advance of said heatexchanger in the direction of gas flow, which method comprises: pe-

riodically spraying the surfaces of said heat absorbing elements duringoperation thereof with an aqueous solution containing sodium'hydrate toform anon-catalytic surface thereon.

7. The method of minimizing corrosion by acof a heat exchanger undercircumstances wherein combustion gases containing are subjected tocatalytic action in passing over iron or steel heat absorbing elementsthat are located in advance of said heat exchanger in the direction ofgas flow and heated to a temperature in the range 8009 to 1200 E, whichmethod comprises: periodically spraying the surfaces of said heatabsorbing elements with an aqueous suspension containing lime hydrate toform a non-catalytic coating thereon.

8. The method of minimizing corrosion by action of sulphuric acids onthe metallic surfaces of a heat exchanger under circumstances whereincombustion gases containing 80: are subjected to catalytic action inpassing over iron or steel superheater elements that are located inadvance" of said heat exchanger in the direction of gas flow and heatedto a temperature in the range 800 to 1200 E, which method comprises:periodically spraying the surfaces of said superheater elements duringoperation thereof with an aqueous suspension vcontaining lime hydrate toform a non-catalytic coating thereon.

9. The method of minimizing corrosion by action of sulphuric acid on themetallic surfaces of a heat exchanger under circumstances whereincombustion gases containing 80: are subjected to catalytic action inpassing over hot iron or steel heat absorbing elements that arelocatedin advance of said heat exchanger in the direction of gas flow,which method comprises: periodically spraying the surfaces of said heatabsorbing eleof said heat exchanger in the direction of gas how andheated to a temperature in the range 800 to 1200 F., which methodcomprises: periodically spraying the surfaces of said. superheaterelements during operation thereof with an aqueous solution containingvery fine puiverized fuel ash. I

WALTER F. HARIDW.

